1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) with high write reproducibility and high write efficiency.
2. Description of the Related Art
Magnetic random access memories have been studied and developed as ultimate nonvolatile high-speed, large-capacity memories.
GMR (Giant Mangeto-Resistance) elements or MTJ (Magnetic Tunneling Resistance) elements are known as memory cells of magnetic random access memories. For data read, a memory using an MTJ element can ensure a larger signal amount than a memory using a GMR element. At present, magnetic random access memories using MTJ elements have enthusiastically been studied and developed.
FIG. 1 shows a cell array structure of a magnetic random access memory using an MTJ element.
The basic structure of the MTJ element is constituted by interposing an insulating layer (tunnel barrier) 2 between two magnetic layers (ferromagnetic layers) 1A and 1B. Cell data is determined by whether the magnetization directions of the two magnetic layers 1A and 1B are parallel or antiparallel. “Parallel” means that the magnetization directions of the two magnetic layers 1A and 1B are the same, and “antiparallel” means that the magnetization directions of the two magnetic layers 1A and 1B are opposite to each other.
An antiferromagnetic layer 3 fixes the magnetization direction of the magnetic layer 1B. The magnetic layer 1B whose magnetization direction is fixed is called a fixed layer or pinned layer. The magnetic layer 1A whose magnetization direction changes depending on the magnetic field is called a recording layer or free layer.
Data is written by causing a rightward or leftward magnetic field to act on the magnetic layer 1A, as shown in FIG. 2. Data is read out by detecting the resistance value of the MTJ element, as shown in FIG. 3. When the magnetization directions of the MTJ element are parallel, the tunnel resistance of the tunnel barrier of the MTJ element becomes lowest. This state is defined as, e.g., a “0” state. When the magnetization directions of the MTJ element are antiparallel, the tunnel resistance of the tunnel barrier of the MTJ element becomes highest. This state is defined as, e.g., a “1” state.
The most important subject of the magnetic random access memory is to reduce the write current. In the memory using the MTJ element, the write current value is higher than an ideal value (8 to 10 mA), and the write current value greatly varies between bit lines, generating a write error.
To put magnetic random access memories into practical use, the write current value and its variations must be reduced to an allowable level. The current write current value reported in academic societies and the like is about 8 mA for an MTJ element about 0.6 μm wide and about 1.2 μm long.
A magnetic field generated by the write current must be strong enough to reverse the magnetization direction of the recording layer (e.g., NiFe with a thickness of 2 to 5 nm) of the MTJ element. This means that a weaker magnetic field necessary to reverse the magnetization direction of the recording layer of the MTJ element can decrease the write current value.
A magnetic field H necessary to reverse the magnetization direction of the recording layer of the MTJ element is given byH≃4πMs×t/F[OE]  (1)
Ms: saturation magnetization, t: recording layer thickness, F: recording layer width
From relation (1), decreasing the recording layer thickness t can weaken the magnetic field H necessary to reverse the magnetization direction of the recording layer of the MTJ element.
However, ensuring thermal disturbance resistance limits a decrease in the thickness of the recording layer of the MTJ element. Considering processing of the MTJ element, the recording layer thickness t must be increased for a recording layer width F of 0.15 μm or less.
Relation (1) also reveals that the magnetic field H necessary to reverse the magnetization direction of the recording layer of the MTJ element is inversely proportional to the recording layer width F. In the future, the recording layer width F is projected to become smaller along with micropatterning of the MTJ element. This increases the write current value much more.
The current density of a current which can be supplied to wiring has an upper limit. This upper limit is 1×107 [A/cm2] for Cu wiring. Since the sectional area of wiring decreases along with micropatterning of the MTJ element, the upper limit becomes lower. As a result, the magnetic field H necessary to reverse the magnetization direction of the recording layer cannot be generated.
A newly developed technique in this situation is a yoke-attached wiring technique.
In the yoke-attached wiring technique, a wiring line (e.g., Cu) as a write line is covered with a soft magnetic material (yoke material) such as NiFe in at least a region where an MTJ element exists. This technique can efficiently concentrate the magnetic field on the MTJ element, reducing the write current value.
At present, it has been reported in academic societies and the like that a memory to which the yoke-attached wiring technique is applied exhibits a write efficiency double that of a memory to which this technique is not applied.
As is apparent from the experimental results shown in FIG. 4, compared to the write current in a memory to which the yoke-attached wiring technique is not applied, a write current half that value is large enough to reverse the magnetization direction of the recording layer of the MTJ element in the memory to which the technique is applied.
A magnetic random access memory to which the yoke-attached wiring technique is applied can reduce the write current value. However, the write current value and its variations are still large for practical use of the magnetic random access memory.
For example, a write line covered with a yoke material was examined by experiments and computer simulation. The write efficiency almost doubled, but disturbs (write errors in half-selected cells) increased.
To solve this problem and reduce the write current, the following three points must be examined.
{circle around (1)} Write Selector Transistor
For example, write lines are individually arranged for MTJ elements, and the current is supplied to only the write line of a selected MTJ element. This technique can effectively decrease disturbs.
{circle around (2)} Covering of Entire Surface of Write Line
The entire surface (upper, lower, and side surfaces) of a write line is covered with a yoke material. By covering the write line with the yoke material, the write efficiency can be further increased.
{circle around (3)} Exchange Coupling
The recording layer (magnetic material) of an MTJ element is brought into contact with a yoke material (magnetic material), and exchange-coupled to the yoke material. “The recording layer is exchange-coupled to the yoke material” means that they have a relationship in which the exchange interaction of electron energy acts. Exchange coupling between the MTJ element and the recording layer can contribute to an increase in write efficiency.
FIG. 5 shows an example of the MTJ element of a magnetic random access memory to which all the techniques {circle around (1)}, {circle around (2)}, and {circle around (3)} are applied and a device structure near the MTJ element.
An MTJ element 6 is arranged at the intersection of two write lines 4 and 5 crossing each other. The write line 5 is connected to a write selector transistor 7. The entire surface of the write line 5 is covered with a yoke material (e.g., NiFe) 8. The MTJ element 6 is directly arranged on the yoke material 8 on the upper surface of the write line 5. The magnetic layer 1A of the MTJ element 6 is exchange-coupled to the yoke material 8.
In this case, Ms*t is defined as a magnetic volume for magnetic layer saturation magnetization Ms and a magnetic layer thickness t.
The magnetic volume around the write line 5 will be examined. On the side and lower surfaces of the write line 5, the magnetic volume isΣMsi′×ti′=Ms′×t′.
where Ms′ is the yoke material saturation magnetization, and t′ is the yoke material thickness.
On the upper surface of the write line 5, the magnetic volume isΣMsi×ti=Ms×t+Ms′×t′.where Ms is the recording layer saturation magnetization of the MTJ element, and t is the recording layer thickness of the MTJ element.
Hence, the magnetic random access memory in FIG. 5 satisfies the inequality:ΣMsi′×ti′<ΣMsi×ti.
In the device structure shown in FIG. 5, disturbs can be satisfactorily decreased. However, the write current value can be reduced to only about 1 mA. For the 1-mA write current, the size (channel width) of the write selector transistor through which this write current flows must be about 1 μm. Such write selector transistors are arranged for respective MTJ elements.
If a magnetic random access memory having a memory capacity of 256 Mega bits or more is constructed, the chip size becomes very large. The magnetic random access memory cannot be put into practical use.
The most serious problem is low write reproducibility. More specifically, write may be achieved with a write current of 1 mA. However, repetitive overwrite results in an overwrite failure at a probability of several ten % (write pass rate=about 87%).
This is because the magnetization of the yoke material (magnetic material) which covers the write line remains in the circumferential direction of the write line upon repetitive write, greatly decreasing the magnetic permeability of the yoke material.